The 45° lock miter joint is considered by many woodworkers to be one of the most beautiful, yet difficult, to make. Proper tools, like a router table with: a large work surface, a stiff fence, and a powerful motor spinning a sharp lock miter bit, are a necessity. Furthermore, boards being worked must be flat and well prepared to prevent exit tear-out and chipping. Finally, a high degree of skill in adjusting the height and depth of cut of the lock miter bit is needed since a lock miter cut must be made in a single pass.
A careful examination of the two boards joined together by a well-made 45° lock miter joint indicates that the bit that produced the cuts in the boards must have been perfectly centered between the top and bottom surfaces of each of the boards. Of course, the center of a 45° lock miter bit is located at the midpoint of the bottom edge of the bit's protruding notch cutting blade. Obtaining correspondence between the centers of the boards and the lock miter bit is not easy.
Normal set-up of a router table to align the center of a board with the center of a 45° lock miter bit is painstaking and time-consuming. First, the thicknesses of the boards are measured. Then, the lock miter bit is moved up and down and the fence is moved back and forth so that the center of the bit is positioned at a distance, equivalent to one-half of the thickness of the boards, away from the top of the router table. Also, the fence is set the same distance from the outer edge of the notch-cutting blade of the bit. Unfortunately, any movement of the bit to adjust cutting height has an effect on both cutting depth and height and the same is true with movements of the fence. Thus, the adjustment process becomes one of hit or miss, dependent upon the accuracy of the measuring devices at hand.
Woodworkers that are tenacious and brave enough to produce woodwork containing lock miter joints are rewarded with items that are striking in appearance and strong. Lock miter joints, as is well known, are virtually invisible and cannot be seen from the outside of an object constructed with them. Further, such joints are sturdy since they have a large surface for gluing.